Redundant residential power sources

ABSTRACT

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for controlling a battery power source. In one aspect, a system includes a first MOSFET having a first gate, a first source, and a first drain. A second MOSFET having a second gate, a second source, and a second drain. The first source is connected to the second source, and the second drain is coupled to a ground. A control circuit connected to the first gate and the second gate and that provides control signals to the first gate and the second gate that cause the first and second MOSFETS to operate in saturation regions during a first operational state to cause the first power source to discharge and the first MOSFET operates in a linear region during a second operational state to limit a charging current that charges the first power source.

BACKGROUND

This specification relates to bi-directional switches and disconnectsfor redundant power systems.

Many redundant commercial and residential power systems include a powersource (e.g., AC grid, solar power, wind power, etc.) and a back-uppower source (e.g., battery, battery bank, generator, etc.). The powersource can be an AC or DC power source that provides power to a load.The back-up power source can include an inverter to convert DC power toAC power to provide AC power to the load. The power systems supply powerto critical and non-critical loads and the system ensures that in theevent the power source loses functionality, the back-up power sourceprovides power to continue the load's operation and functionality.

SUMMARY

In general, one innovative aspect of the subject matter described inthis specification can be embodied in systems and methods that include afirst MOSFET having a first gate, a first source, and a first drain. Asecond MOSFET having a second gate, a second source, and a second drain.The first source is connected to the second source, and the second drainis coupled to a ground. A first power source having a first powerterminal and a second power terminal, where the first power terminal isconnected to the first drain and second power terminal is connected to aDC bus. A second power source having a third power terminal and a fourthpower terminal, where the third power terminal is connected to theground and the fourth power terminal is connected to the DC bus. Acontrol circuit connected to the first gate and the second gate and thatprovides control signals to the first gate and the second gate thatcause the first and second MOSFETS to operate in saturation regionsduring a first operational state to cause the first power source todischarge and the first MOSFET operates in a linear region during asecond operational state to limit a charging current that charges thefirst power source. Other embodiments of this aspect includecorresponding systems, apparatus, and computer programs, configured toperform the actions of the methods.

Particular embodiments of the subject matter described in thisspecification can be implemented so as to realize one or more of thefollowing advantages. The systems and methods disclosed hereinfacilitate a transition of providing power from a primary power sourceto a secondary power source using inexpensive MOSFET switches instead ofan active battery converter. By utilizing a bi-directional switch, thesystems and methods can discharge (e.g., provide power) a secondarypower source and can charge the secondary power source. In doing so,charging to discharging and discharging to charging transitions areautomatically achieved by voltage and current controllers. In addition,the bi-directional switch can disconnect the secondary load from a powerbus in the event of a fault on the power bus. The transition fromcharging to discharging to disconnecting is achieved with voltage andcurrent controllers each independently regulating current and voltageduring charging.

The details of one or more embodiments of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example redundant power system.

FIG. 2 is a block diagram of the example redundant power system thatincludes a switch and a controller.

FIG. 3 is a circuit diagram of an example controller.

FIG. 4 is a diagram illustrating example current and voltage waveformsrepresentative of various operations of the redundant power system.

FIG. 5 is flow diagram of a redundant power system operation.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

Overview

A redundant power system enables a load to receive uninterrupted powerif a primary power source is unable to independently provide power tothe load. Typically, the redundant power system includes a power sourceand/or a back-up power source to power the load by transitioning thesupply of power from the primary power source to the power source and/orback-up power source. A bi-directional switch that is activated by acontroller can effectuate the back-up power source during a loss offunctionality by the primary power source and/or the power source. Inaddition, the bi-directional switch also enables a reverse flow ofcurrent to charge or re-charge the back-up power source.

The bi-directional switch also operates as a disconnect switch that canisolate the back-up power source from the load. For example, in theevent of a fault within the system the controller operates thebi-directional switch to disconnect the back-up power supply from theload mitigating an over-current situation that may damage the back-uppower source.

The redundant power system can include a power conversion architecturefor a residential home that is based on one or more energy sourcescoupled together with a battery system. For example, the load may be anAC load or a DC load, the primary power source can be the AC grid, andthe power source can be a photovoltaic system, a wind turbine, agenerator, etc. The power source includes an AC power source coupledwith a converter or a DC power source that includes an energy conversionmechanism (e.g., buck-boost transformer). The back-up power source caninclude a battery or some other direct current supplying energy source(e.g., photovoltaic system, wind turbine, etc.) that supplements powerto the redundant power system.

These features and other features will be described in more detailbelow.

FIG. 1 is a block diagram of an example redundant power system 100. Thepower system 100 provides uninterruptable power to a load 114. In theexample that follows, the system 100 will be described in theapplication of a residential power system. However, the system can beapplied in other settings, such as in commercial applications andindustrial applications.

The power conversion architecture for a residential home is based on atleast one energy source couple with a battery system. For example, theredundant power system 100 can include a controller 102, a switch 104, abattery 106, a DC bus 108, a secondary power source 110, aninverter/rectifier 112, and a primary power source 116. Typically, theprimary power source 116 is the main source of power (e.g., AC grid).

The secondary power source 110 can include an energy converter that canconvert AC power to DC power or can transform a DC voltage to adifferent DC voltage. The secondary power source can be a solar powersystem, a wind turbine system, a generator, or any other power deliverysystem of the like. The secondary power source 110 provides DC power tothe DC bus 108.

The inverter/rectifier 112 receives power from the DC bus 108, convertsthe DC power to AC power to supply to a load 114. Here the load 114represents common household electronic devices that are used within ahome, commercial buildings, or utility infrastructure. Theinverter/rectifier 112 can be a rectifier, a solar inverter/rectifier,or any other mechanism that converts AC power to DC power.

In some implementations, the inverter/rectifier 112 can be a rectifierthat converts AC power received from the grid to DC power. For example,the inverter/rectifier 112 may receive power from the AC grid, convertthe power to DC to supply DC power to the DC bus 118. Often a powersystem may have a second harmonic (e.g., 120 Hz) ripple voltage that canexist on the DC bus that is proportional to the current of theinverter/rectifier output power. The second harmonic ripple voltage isalso inversely proportional to the DC bus capacitance. The ripplevoltage can be minimized by increasing the DC bus capacitance orlowering the inverter/rectifier output current.

The battery 106 is a back-up power source that provides direct currentto the DC bus 108 in the event the primary and secondary power source110 fail to independently provide power to the load 114. The battery 106can be a single battery or a group of batteries coupled together, whichever configuration is sufficient to provide enough current to power theload 114. Typically, the battery 106 is a rechargeable battery that candischarge stored energy when it is providing DC power and can charge byaccepting a direct current to replenish stored energy within thebattery. For example, the battery 106 may receive DC power that issupplied by the AC grid and rectified by the inverter/rectifier 112.

The battery 106 can also operate in response to an additional powerdemand. In some instances, the load may require more power than can besupplied by the primary and/or secondary power source. In such an event,the battery 106 may supply supplemental power to the load 114. Inaddition, the battery 106 can be operational for charging anddischarging for efficient energy flow that maintains a constant voltageand/or current supplied to the load 114.

The battery 106 works in conjunction with a switch 104 during chargingand discharging of the battery 106. The switch 104 enables abi-directional flow of current to and from the DC bus 108. For example,when the battery is discharging, current is flowing out of the battery.Conversely, when the battery is charging, current is flowing into thebattery. In addition, the switch can also disconnect the battery 106from the bus if an event occurs that may adversely affect the battery,such as a fault being detected on the bus.

To manipulate the various functions of the switch 104, the switch 104 iscontrolled by the controller 102. The controller 102 operates the switchto enable current to flow in the direction determined by the redundantpower system 100. For example, the controller 102 can provide a controlsignal to the switch that dictates an operational state of the switch(e.g., forward current flow, reverse current flow, no current flow,etc.) In some implementations, the controller 102 can be an analogcontroller or a digital (e.g., processor, electronic control system,etc.). Further details regarding the switch 104, the controller 102, anddifferent implementations of the switch 106 and the controller 102 aredescribed in connection with FIGS. 2, 3, and 4 below.

FIG. 2 is a block diagram of the example redundant power system 100 thatincludes a switch 104 and a controller 102. In one implementation, theswitch 104 is a bi-directional switching circuit 104 that includes twotransistors connected in series. For example, the two transistors can beMOSFETs 202, 204. By operating the MOSFETs 202, 204 in their variousoperational states (e.g., linear region, saturation region, cut-offregion, etc.), the MOSFETs function to direct the flow of current asdetermined by the redundant power system controller 102.

As shown in FIG. 2, the bi-directional switching circuit 104 includes afirst MOSFET 202 that has a first gate 206, a first drain 208, and afirst source 210. The bi-directional switching circuit 104 also includesa second MOSFET 204 having a second gate 212, a second source 214, and asecond drain 216. In one implementation, the first source 210 of thefirst MOSFET 202 is connected to the second source 214 of the secondMOSFET 204, and the second drain 216 of the second MOSFET 204 is coupledto a ground 218. In other implementations, the first drain 208 of thefirst MOSFET 202 can be connected to the second drain 216 of the secondMOSFET 206.

The redundant power system 100 includes a first power source (e.g., thebattery 106) having a first power terminal 220 and a second powerterminal 222. In one example, the first power terminal 220 is connectedto the first drain 208 and the second power terminal 222 is connected toa DC bus 108. As previously described, the first power source providespower to the DC bus 106 when the first power source is discharging.Generally, the battery 106 discharges when the second power source(e.g., the primary power source and/or the secondary power source) 110is unable to independently power the load 114.

The redundant power system also includes a second power source (e.g.,the secondary power source) 110 having a third power terminal 224 and afourth power terminal 226. In one implementation, the third powerterminal 224 is connected to the ground 218 and the fourth powerterminal 226 is connected to the DC bus 108. Generally, the second powersource 110 is the secondary power source that can provide power to theload 114 for the redundant power system 100.

The operational state of the MOSFETS 202, 204 is typically controlled bya control circuit (e.g., the controller 102) that is connected to thefirst gate 206 and the second gate 212. The controller 102 providescontrol signals to the first gate 206 and the second gate 212. In theexample shown, the gates 206 and 212 are coupled to the same controlsignal; however, depending on the controller design, the gates can beoperated separately and the MOSFETs 202 and 204 can be operated indifferent states so long as the charging and discharging characteristicdescribed below are realized.

In some implementations, the control signal causes the first and secondMOSFETs 202, 204 to operate in saturation regions during a firstoperational state to cause current to flow (e.g., 0˜100 A) from thefirst power source (e.g., the battery 106). The controller can alsoprovide control signals to the first and second MOSFETs 202, 204 tocause the first MOSFET 202 to operate in a linear region during a secondoperational state (i.e., the linear region). Operating in the linearregion limits the amount of charging current that is provided to thefirst power source during the charging process. Limiting the chargingcurrent that is received by the first power source enables a controlledcharging of the first power source and ensures that the current drawdoes not exceed the current capability of the primary source.

Operational states of the switching circuit 104 is dependent, in part,on the DC Bus voltage. Upon failure of the second power source 110 toindependently provide power to the load, the voltage on the DC bus 108diminishes. The control circuit (e.g., the controller 102) determinesthat the first operational state occurs when the voltage of the DC bus108 is less than a voltage of the first power source (e.g., the battery106) as measured between the first power terminal 220 and the secondpower terminal 222. The control circuit sends a control signal to theMOSFETs that cause the MOSFETs to operate in the first operational state(e.g., saturation region). Operating in the saturation region enablesthe first power source to begin powering the load and deplete the energystored within the first power source.

The control circuit (e.g., the controller 102) also determines thesecond operational state occurs when the voltage of the DC bus 108 isgreater than the voltage of the first power source as measured betweenthe first power terminal 220 and the second power terminal 222. Thevoltage of the DC bus 108 being higher than the voltage of the firstpower source is usually indicative of the second power source (e.g., thesecondary power source 110) and/or the primary power source 116returning to operation. After the battery discharges and the secondpower source 110 returns to operation, the second operational state(e.g., linear mode) enables current to be delivered to the first powersource to charge the first power source. Operating the MOSFETs in linearmode provides a current controlling mechanism that limits the amount ofcurrent delivered to the first power source.

The control circuit (e.g., the controller 102) can also detect a faultcondition on the DC bus 108. To protect the first power source fromdamage caused by a potential over current situation, the control circuitsends a control signal to operate the first MOSFET and the second MOSFETin a cut-off mode. The cut-off mode opens the portion of the circuitthat is connected to the first power source which disables the flow ofcurrent to the first power source.

In some implementations (not shown), the redundant power system can bescaled to include multiple sets of MOSFETs. For example, the redundantpower system 100 can include two or more sets of MOSFETs connected inparallel between the battery 106 and ground 218. In someimplementations, each pair of MOSFETs can have a dedicated controller todictate the operational states for each pair of MOSFETs. In otherimplementations, each individual MOSFET can have its own dedicatedcontroller to dictate the operational state for an individual MOSFET.The controllers can be communicatively linked and determine collectivelythe operational state for each of the MOSFETs or each controller canindependently determine the operational state for its MOSFET.

In some implementations, the controller 102 can include an analogcircuit or a digital circuit. For example, the control circuit (e.g.,the controller 102) can include a processing device that is coupled tothe first and the second gate 206, 212 and provides control signals tothe first and second MOSFETs 202, 204 to operate the MOSFETs in thedifferent operational states.

One example implementation using analog components is shown in FIG. 3,which is a circuit diagram of an example controller 102. The controlcircuit of the controller 102 includes a charging voltage controlcircuit 302 and a charging current control circuit 306. The chargingvoltage control circuit 302 includes a first amplifier 304 and an outputof the first amplifier is coupled to the first gate 206 and the secondgate 212 through a diode D1. The charging voltage control circuit 302can sense the voltage of the battery 106 and compares it to a referencevoltage, e.g., a voltage that is less than the voltage of the DC bus.When the battery voltage is much less than the reference voltage, theoutput Vu2 will be high. When the battery voltage is much higher thanthe reference voltage, the output Vu2 will be low; otherwise Vu2 will beadjusted to regulate the battery voltage. Further details about thereference voltage will be described in connection with FIG. 4.

The charging current control circuit 306 includes a second amplifier 308that has an output that is coupled to the first gate 206 and the secondgate 212. The charging current control circuit 306 senses the batterycurrent by means of the current sense resistor R1. Similar to thebattery voltage, when the battery current is much less than thereference current, the output of the Vu1 will be high. When the batterycurrent is much higher than the reference current, the output of the VU1will be low; otherwise VU1 will be adjusted to limit or regulate thebattery current.

Together the outputs Vu1 and Vu2 cooperative control charging anddischarging of the battery. More specifically, the output of u1regulates or limits the charging current by adjusting the gate-to-sourcevoltage of the MOSFETs 206 and 212 to operate in the linear operationalregion during charging. The output of u2 is coupled by a diode (D1) sothat the battery voltage is regulated at a certain level.

In some implementations, the operations of the controller 102 may befurther subject to a battery management system 310 that would govern,according to one or more optimization constraints, when the battery candischarge or charge. The battery management system 310 can override thecontroller 102 to either enable or disable charging and discharging ofthe battery dependent upon the optimization constraints. For example,during peak power times, when it may be more expensive to consume power,the battery management system 310 may suspend or limit the batterycharging operation.

The battery management system 310 can govern other aspects of theredundant power system 100. In some implementations, the batterymanagement system 310 can override operations of the system to dictatevoltage levels, current draw, operational state of the switch (e.g.,MOSFETS 202, 204), and other operations of the like. For example, thebattery management system 310 may determine, due to load constraints orsome other system attribute, to adjust (e.g., lower or raise) thevoltage the of the DC bus 108 by sending operational instructions to theinverter 112.

Operation of the circuit of FIG. 3 is described with reference to FIG.4, which is a diagram illustrating example current and voltage waveformsrepresentative of one example of the redundant power system's variousoperations. The current and voltage waveforms detail one example of theredundant power system operations for providing uninterruptable power toa load 114.

Referring to FIG. 4, at time t0, the redundant power system is operatingwith the secondary power source 110 independently providing power to theinverter/rectifier 112. The DC bus voltage is maintained at a highervoltage than the battery voltage, Vbat, during the charging mode or whenthe secondary power source 110 is independently providing power to theinverter/rectifier 112. The battery is fully charged and thus thecurrent, Ibat, is almost zero (or some nominal leakage or inactivevalue) since the battery is not providing power to theinverter/rectifier 112. The output voltage of the charging currentcontroller 306, VU1, is high (e.g., significantly greater than a “zero”output, such as at or near the positive rail).

The output of the charging voltage controller 302, Vu2, is low. Becausethe battery voltage is at or above the reference voltage value, thecharging voltage controller is regulating the battery voltage. In someimplementations, the reference voltage value is a reference value thatis less than the specified DC bus voltage since the DC bus has the 120Hz (2^(nd) harmonic) ripple voltage 418. As will be explained below, thereference value being less than the specified DC bus values enablesregulation of the battery voltage during the last portion of the batteryrecharging phase, which is referred to as a third operational state.

Time t1 illustrates the secondary power source 110 failing toindependently supply power to the inverter/rectifier 112. During thistime, the battery is discharging. In some implementations, the batterycan deliver power to the inverter/rectifier 112 without an failure ofthe primary and/or secondary power sources. The controller 102 monitorsa voltage of the DC bus 108 indirectly by means of monitoring thebattery 106 voltage. The battery voltage falling below a referencevoltage is indicative of the DC bus being at or below the batteryvoltage. The voltage for the DC bus is typically provided by thesecondary power source 110, but as shown at t1 the DC bus voltage hasreached the battery voltage. Thereafter, the controller determines thatthe voltage of the DC bus 108 is below a battery voltage (402) due tothe battery voltage falling below the reference voltage. In response todetermining that the battery voltage is below the reference voltage andthe battery charging current is less than a reference current, thecontroller 102 provides a control signal to operate the first and secondMOSFETS 202, 204 in saturation regions to cause the battery to dischargeonto the DC bus 108.

As shown at time t1 in the circuit of FIG. 3, the output of the chargingcurrent controller, Vu1, remains high and the charging voltagecontroller, Vu2, switches its output to high. This results is a highgate voltage at Vgate (404). The high Vgate voltage drives the MOSFETs202, 204 to a saturation region allowing current to flow from thebattery 106 to the DC bus 108. Between the times t1 and t2, the battery106 is discharging as indicated by the decreasing battery voltage Vbat(406). Accordingly, the battery current, Ibat, is negative (408),indicating power being supplied by the battery 106 to theinverter/rectifier 112. The negative value of Ibat also maintains theoutput VU1 high, as the negative results in a current sense signal thatis less than the reference current voltage signal. Thus, the time fromt1 to t2 is a first operational state in which the first and secondMOSFETS to operate in saturation regions and the first operational stateoccurs when the voltage of the DC bus is less than a voltage of thebattery as measured between the first power terminal and the secondpower terminal of the battery.

During this time, the power loss of MOSFET 204 determined by theon-resistance R_(M2) of the MOSFET 204 during discharging:

P _(Loss) =I _(Bat,dischg) ² ·R _(M2)

The time t2 represents a second operational state during which the firstMOSFET operates in a linear region to limit a charging current thatcharges the battery. This state is caused by the combination of thevoltage of the DC bus being greater than the voltage of the battery andthe voltage of the battery being less than the reference voltage inputamplifier 304, which results in a battery current regulated at areference current. In particular, in the example circuit of FIG. 3, thiscondition of the DC bus voltage being higher than the battery voltagecauses the battery to charge

Referring to FIG. 4, the time t2 illustrates the secondary power source110 returning to operation as the DC bus voltage, Vbus, is higher thanthe battery voltage, Vbat (410). In response to the DC bus voltage beinghigher than the battery voltage but lower than the reference voltage,the controller 102 cooperatively operates the first and second MOSFETs202, 204 to cause the battery to charge from the DC bus by a controlledcurrent and independent of control of the battery voltage. For example,the controller 102 provides a control signal to the MOSFETs 202, 204 bychanging the output voltage of the charging current controller 306, VU1.The voltage Vgate is determined by a voltage drop across resistor R1that places the first and second MOSFETs 202, 204 in a linear mode,limiting the amount of current provided to the battery.

Operating at least the first MOSFET 202 in linear mode effectuatescharging the battery 106 with limited current. As shown in FIG. 4, Ibatis positive (412), indicating that the battery 106 is receiving currentduring the charging process.

Time t3 illustrates a time when the battery voltage has reached areference voltage and the battery 106 begins to be charged with acontrolled voltage. This is referred to as a third operational state. Inthis state, the nominal voltage of the DC bus (414) is higher than thebattery voltage (416), and the battery voltage is at least equal orclose to the reference voltage. Because the reference voltage is lessthan the specified DC bus voltage, the battery voltage is thuscontrolled to maintain the battery voltage below the DC bus voltage toprevent damage and cycling effects of the battery due to the secondharmonic (e.g., 120 Hz) ripple voltage 418 that is on the DC bus 108.

In operation, the controller 102 senses that the voltage of the DC bus108 meets a reference voltage. In response, the controller 102cooperatively operates the first and second MOSFETs 202, 204 to causethe battery to charge from the DC bus 108 by a controlled voltage andindependent of control of the current. For example, the charging voltagecontroller 302 changes its output voltage, VU2. The resulting decreasesin Ibat causes the output VU1 to go high. The resulting gate voltageVgate, determined by the blocking diode D1 operates the MOSFET 202 inthe linear region, but the battery voltage is regulated by the positivedifference between the battery voltage and the reference voltage.

During the second and third operational state (during charging), MOSFET202 operates in the linear operational region. Power loss of the MOSFETduring charging is mainly determined by

Vbus as,

P _(Loss)=(V _(bus) −V _(bat))×I _(bat,chg)

In some implementations, to lower Ploss during charging, the DC busvoltage Vbus remains close to the battery voltage Vbat. Thus Vbus isadjusted/regulated at slightly higher voltage than the battery voltageVbat by an energy converter and/or an inverter/rectifier.

FIG. 5 is flow diagram of a redundant power system operation. The flowdiagram describes one implementation of providing back-up power to aload 114 by a primary and/or secondary power source 110, a battery 106,a controller 102, and a bi-directional switching converter 104. In someimplementations, the redundant power system 100 can be used as anuninterruptable power supply. In other implementations, the redundantpower system 100 can be used as a back-up power supply to supplysupplemental power, additional power, etc. to the inverter/rectifier112. The process includes a processing device (e.g., analog controller,digital controller, etc.) that is configured to monitor a voltage of aDC bus 108 (502). As previously described, when the primary and/orsecondary power source 110 is independently powering the load 113, thevoltage for the DC bus 108 is being provided by a primary and/orsecondary power source.

The process determines that the battery voltage and current are below areference voltage and a reference current and in response operating thefirst and second MOSFETs 202, 204 in saturation regions (504). Operatingthe first and second MOSFETs 202, 204 in saturation regions closes theportion of the circuit that is connected to the battery 106 enabling thebattery to discharge current onto the DC bus providing back-up power tothe load 114.

Upon the primary and/or secondary power source 110 returning tooperation, the process determines that the voltage of the DC bus 108 isabove the battery voltage and the battery voltage is below a referencevoltage, and in response cooperatively operates the first and secondMOSFETs 202, 204 to cause the battery to charge by a limited chargingcurrent (506). The battery receives current from the DC bus 108 by acontrolled current and independent of control of the battery voltage.The first and second MOSFETs 202, 204 operate in a linear mode toprovide a current limiting mechanism that controls the amount of currentreceived by the battery. In some implementations, during this portion ofthe charging process, the charging voltage controller does not controlthe voltage of the battery (e.g., Vbat). For example, the voltage of thebattery 106 is enabled to recover at an uncontrolled rate according tothe amount of current that is delivered to the battery, and is onlylimited by the charging current.

The battery voltage recovers until it is determined that the voltage ofthe battery meets the reference voltage, the battery current is below areference current, and in response the process 500 cooperativelyoperating the first and second MOSFETs 202, 204 to cause the battery tocharge from the DC bus 108 by a controlled voltage and independent ofcontrol of the current (508). As previously described, the chargingvoltage controller 302 adjusts the voltage of the battery 106 byadjusting the gate voltage of the first MOSFET 202. Vbat is maintainedat a voltage lower than Vbus to ensure the second harmonic ripplevoltage that is present on the DC bus does not have an adverse effect onthe battery 106.

The example implementations above are described in the context of ananalog circuit that cooperative controls gate voltages of the MOSFETSbased on a battery voltage and charger current. Other appropriate analogor digital control circuits can be used to realize the functionaloperational states described above. For example, in otherimplementations, the control circuit can drive the MOSFETS 202 and 204separately. Additionally, the controller 102 can be implemented as aprocessing device (not shown) to control the instructions provided tothe bi-directional switching circuit 104.

Embodiments of the subject matter and the operations described in thisspecification can be implemented in digital electronic circuitry, or incomputer software, firmware, or hardware, including the structuresdisclosed in this specification and their structural equivalents, or incombinations of one or more of them.

The operations described in this specification can also be implementedas operations performed by a data processing apparatus on data stored onone or more computer-readable storage devices or received from othersources.

The term “data processing apparatus” encompasses all kinds of apparatus,devices, and machines for processing data, including by way of example aprogrammable processor, a computer, a system on a chip, or multipleones, or combinations, of the foregoing. The apparatus can includespecial purpose logic circuitry, e.g., an FPGA (field programmable gatearray) or an ASIC (application-specific integrated circuit). Theapparatus can also include, in addition to hardware, code that createsan execution environment for the computer program in question, e.g.,code that constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, a cross-platform runtimeenvironment, a virtual machine, or a combination of one or more of them.The apparatus and execution environment can realize various differentcomputing model infrastructures, such as web services, distributedcomputing and grid computing infrastructures.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform actions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., a FPGA (field programmable gate array) or an ASIC(application-specific integrated circuit).

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyfeatures or of what may be claimed, but rather as descriptions offeatures specific to particular embodiments. Certain features that aredescribed in this specification in the context of separate embodimentscan also be implemented in combination in a single embodiment.Conversely, various features that are described in the context of asingle embodiment can also be implemented in multiple embodimentsseparately or in any suitable subcombination. Moreover, althoughfeatures may be described above as acting in certain combinations andeven initially claimed as such, one or more features from a claimedcombination can in some cases be excised from the combination, and theclaimed combination may be directed to a subcombination or variation ofa subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems cangenerally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular embodiments of the subject matter have been described.Other embodiments are within the scope of the following claims. In somecases, the actions recited in the claims can be performed in a differentorder and still achieve desirable results. In addition, the processesdepicted in the accompanying figures do not necessarily require theparticular order shown, or sequential order, to achieve desirableresults. In certain implementations, multitasking and parallelprocessing may be advantageous.

What is claimed is:
 1. A system, comprising: a first MOSFET having afirst gate, a first source, and a first drain; a second MOSFET having asecond gate, a second source, and a second drain, wherein the firstsource is connected to the second source, and the second drain iscoupled to a ground; a first power source having a first power terminaland a second power terminal, wherein the first power terminal isconnected to the first drain and second power terminal is connected to aDC bus; a second power source having a third power terminal and a fourthpower terminal, wherein the third power terminal is connected to theground and the fourth power terminal is connected to the DC bus; and acontrol circuit connected to the first gate and the second gate and thatprovides control signals to the first gate and the second gate thatcause: the first and second MOSFETS to operate in saturation regionsduring a first operational state to cause the first power source todischarge; and the first MOSFET operates in a linear region during asecond operational state to limit a charging current that charges thefirst power source.
 2. The system of claim 1, wherein: the firstoperational state occurs when a reference voltage is higher than avoltage of the first power source as measured between the first powerterminal and the second power terminal; and the second operational stateoccurs when: the voltage of the DC bus is greater than the voltage ofthe first power source as measured between the first power terminal andthe second power terminal; and the voltage of the first power source isless than a reference voltage.
 3. The system of claim 1, wherein thecontrol circuit provides further control signals to the first gate andthe second gate that causes the first MOSFET to operate in the linearregion during a third operational state to regulate a voltage of thefirst power source.
 4. The system of claim 1, wherein the controlcircuit the third operational state occurs when: the voltage of the DCbus is greater than the voltage of the first power source as measuredbetween the first power terminal and the second power terminal; and thevoltage of the first power source is less than the reference voltage. 5.The system of claim 1, wherein the control circuit comprises: a chargingvoltage control circuit, the charging voltage control circuit includinga first amplifier, wherein an output of the first amplifier is coupledto the first gate and the second gate; and a charging current controlcircuit, the charging current control circuit including a secondamplifier, wherein an output of the second amplifier is coupled to thefirst gate and the second gate.
 6. The system of claim 1, wherein thecontrol circuit comprises: a charging voltage control circuit having anoutput of the charging voltage control circuit coupled to the firstgate; and a charging current control circuit having an output of thecharging current control circuit coupled to the second gate.
 7. Thesystem of claim 1, wherein the control circuit comprises a processingdevice coupled to the first and the second gate and provides controlsignals to the first and second MOSFETs to operate the MOSFETs indifferent operational regions.
 8. The system of claim 1, wherein thefirst MOSFET is one of a plurality of first MOSFETS and the first MOSFETincludes a dedicated control system.
 9. The system of claim 1, whereinthe second MOSFET is one of a plurality of second MOSFETS and the secondMOSFET includes a dedicated control system.
 10. The system of claim 1,further comprising a power management system coupled to the controlcircuit providing operational signals to the control circuit, theoperational signals including operational instructions for the first andsecond MOSFETS based on optimization constraints.
 11. A method,comprising: monitoring a voltage of a DC bus, the voltage for the DC busbeing provided by a primary power source; determining that the voltageof the DC bus is below a battery voltage and in response operating firstand second MOSFETS in saturation regions to cause the battery todischarge onto the DC bus; determining that the voltage of the DC bus isabove the battery voltage and the battery voltage is below a referencevoltage and in response cooperatively operating the first and secondMOSFETs to cause the battery to charge from the DC bus by a controlledcurrent and independent of control of the battery voltage; anddetermining that the voltage of the DC bus is above the battery voltageand that the voltage of the battery meets the reference voltage and inresponse cooperatively operating the first and second MOSFETs to causethe battery to charge from the DC bus by a controlled voltage andindependent of control of the current.
 12. The method of claim 11,further comprising: detecting a fault condition on the DC bus: operatingthe first MOSFET and the second MOSFET in a cut-off mode, wherein thecut-off mode opens a portion of a circuit that is connected to thebattery disabling the flow of current within the portion of the circuit.13. The method of claim 11, wherein the DC bus voltage is regulated byan energy converter.
 14. The method of claim 11, wherein cooperativelyoperating the first and second MOSFETs to cause the battery to chargefrom the DC bus further comprises: operating the first and secondMOSFETs in a linear mode limiting the amount of current provided to thebattery; determining that the voltage for the DC bus meets a referencevoltage; and regulating the battery voltage using a charging voltagecontrol circuit, wherein regulating the battery voltage comprisesmodulating a voltage between the first gate and the first source of thefirst MOSFET.
 15. The system of claim 11, wherein the first MOSFET isone of a plurality of first MOSFETS and the first MOSFET includes adedicated control system.
 16. The system of claim 11, wherein the secondMOSFET is one of a plurality of second MOSFETS and the second MOSFETincludes a dedicated control system.
 17. The system of claim 11, furthercomprising a power management system coupled to the control circuitproviding operational signals to the control circuit, the operationalsignals including operational
 18. A system, comprising: a processingdevice configured to: monitor a voltage of a DC bus, the voltage for theDC bus being provided by a primary power source; determine that abattery voltage is below a reference voltage and in response operatingfirst and second MOSFETS in saturation regions to cause the battery todischarge onto the DC bus; determine that the voltage of the DC bus isabove the battery voltage and the battery voltage is less than areference voltage that is less than a specified DC bus voltage and inresponse cooperatively operating the first and second MOSFETs to causethe battery to charge from the DC bus by a controlled current andindependent of control of the battery voltage; and determine that thevoltage of the DC bus meets the reference voltage and in responsecooperatively operating the first and second MOSFETs to cause thebattery to charge from the DC bus by a controlled voltage andindependent of control of the current.